Unlockable weapons based on acheivements which seem to be based around how long you play the game more than how good you are will just work to further build the wall between new and veteran players.
Veterans will play until they have all the unlocks, and then a 'best' combination will appear, meaning each class end up only using one type of weapon anyway.
Newbies will join, have no idea whats going on, be utterly confused, get beaten, and leave.
This update could be great for hardcore players for a little while, untill they settle into a rut of best weapons from the selection, and new players will be alienated.
Actually, these are just my thoughts of some possibilities, but they could be right, you never know. If it aint broke dont fix it. Having strict classes works. As soon as you start to blurr the lines between classes (giving snipers decent close range guns, pyros anything other than a flamer, scout any more than a short range gun, etc) you loose what defines the game.